Electrodes for measuring biopotential are used extensively in modern clinical and biomedical applications. These applications encompass numerous physiological tests including electrocardiography (ECG/EKG), electroencephalography (EEG), electrical impedance tomography (EIT), electromyography (EMG) and electro-oculography (EOG). The electrodes for these types of physiological tests function as a transducer by transforming the electric potentials or biopotentials within the body into an electric voltage that can be measured by conventional measurement and recording devices.
Most physiological electrodes for such applications are placed on the skin. The skin presents a layered architecture. Generally, the outer skin layer, stratum corneum, forms the primary barrier of the body. It constantly is renewing itself and consists of dry, dead cells, which dramatically influences electrical isolation characteristics, i.e., creates high electrical impedance, compared to living cells. It also renders the administration of drugs on the skin less effective. Typically, the skin must be prepared prior to the application of electrodes or treatment.
Below the stratum corneum are several other layers, including the stratum germinativum. The stratum germinativum, is the area where the cells divide, grow, and are displaced outward to the stratum corneum. Since the stratum germinativum is composed of living cells that predominately consist of liquid, this layer of the skin is an electrically conducting tissue comparable to an electrolyte. The stratum corneum, further is very thin and uniform in most regions of the body surface ranging from 13-15 μm with a maximum of about 20 μm. The dermis, which is below the stratum germinativum, contains vascular and nervous components as well as sweat glands and hair follicles and is also electrically conducting. It is in the dermis that pain has its origins.
FIG. 1 shows the sensing electrode used in a conventional biopotential measurement system. In order to get over the high impedance of the Stratum Corneum 12 with bad electric conductivity, it is necessary to reduce or remove the Stratum Cornuem (including shaving of the area) or use electric conductive adhesive 18 or similar material under the sensing electrode 10 to moisten the Stratum Corneum 12 so as to enhance the conductive effect. The whole measuring effect, however, is much limited, due to drying, irritation, movement and other well documented problems. Similar or additional problems are associated with “dry” electrodes. Therefore, U.S. Pat. No. 6,334,856 discloses a microneedle device 20 shown in FIG. 2 as a sensing electrode, wherein the microneedle 22 penetrates the Stratum Corneum 12 and gets into the Stratum Germinativum 14 to measure biopotential signals. Because the Stratum Germinativum 14 consists of live cells and has a good electric conductivity, the microneedle device 20 requires no electric conductive adhesive to obtain better measuring results. Moreover, the length of the microneedle 22 can be controlled not to get into the dermis 16, hence not causing pain or bleeding to the human body. Because the microprobe has the above advantages, it has replaced the conventional sensing electrode and been widely used in the biopotential measurement systems.
When the microneedle device 20 is used as a sensing electrode, a certain external force is applied to let the microneedle 22 puncture the Stratum Corneum 12 and get into the Stratum Germinativum 14. Because the skin tissue will continually push outwards the microneedle 22 and the microneedle device 20 is of a conical shape that has a wide bottom and a narrow top and hence has no stabilization capability, the actions of the muscles under the skin will easily loosen the probe 22. Therefore, it is necessary to use a fixing tool such as adhesive tape to fix the microneedle device 20 on the skin. Since the skin tissue is very soft, the fixing effect of the microneedle device 20 varies, hence affecting the position where the probe 22 gets into the skin and making the quality of signal measurement hard to control. In consideration of this inherent problem, U.S. Pat. No. 6,690,959 discloses a microprobe structure having a probe with an inverted hooked pinpoint to let the microprobe “lodge” within the skin. But forming an inverted hook on a microprobe structure at the micrometer level is very difficult to manufacture utilizing current technique, which the patent did not disclose a feasible manufacturing method either. Although PCT Application No. WO 01/52731 discloses a process for deriving a more durable barbed spike, both probes suffer from potential breakage of the barb or damage to the skin due to probe movement caused by the barb.
Accordingly, the present invention aims to propose a microprobe array structure having a self-stabilization function and capable of measuring biopotential signals and a method for manufacturing the same.